Sputter deposition and ion beam deposition (IBD) are familiar methods for depositing thin film materials. These deposition processes require deposition on substrates with particular topographical features that affect the distribution and properties of deposited material across the substrate. For example, lift-off deposition processes in which thin films are deposited over a pattern of photoresist features are used in many important thin film device fabrication processes.
IBD is particularly well suited for lift-off deposition processes due to some unique advantages of the process, including low process pressures and directional deposition. As a result, the lift-off step is extremely clean and repeatable down to critical dimensions less than 0.5 microns. Primarily because of these advantages, IBD has become the dominant method for depositing stabilization layers for thin film magnetic heads as a lift-off step is required subsequent to the deposition of the stabilizing material. In addition to good lift-off properties, IBD films have extremely good magnetic properties. The substrate may be tilted to different angles to optimize the properties of the IBD deposited film and rotated to average out non-uniformities introduced by the tilting.
With reference to FIG. 1, an IBD system generally includes a deposition gun 10 that directs an energized beam 12 of ions to a target 14 of material to be deposited. The ion beam 12 sputters material from a finite, well-confined source region on the target 14 to generate a beam 16 of sputtered target material. A substrate 18 is held on a fixture 20 and positioned so that the beam 16 impinges the substrate 18. The target 14 is approximately the size of substrate 18, which is located the equivalent of a few substrate diameters away from the target 14. The fixture 20 is configured to tilt the normal to the surface of substrate 18 at an angle θ relative to the direction of the deposition flux 16 and to continuously rotate the substrate 18 about the surface normal.
The divergence angle of the beam 16 depends on the geometrical relationship between the target 14 and substrate 18. One contribution to the divergence angle arises because the ion beam 12 is focused on the target 14 to prevent ion beam sputtering of nearby components in the process chamber. Another contribution to the divergence angle originates from the target-to-substrate distances that are limited due to the deposition rate reduction.
Beam divergence in IBD systems cause asymmetrical shadowing of the substrate surface by the features projecting from the substrate surface, such as the features characterizing a photoresist pattern. This causes the deposited material to have an asymmetric deposition profile relative to the features, which reduces the area over which lift-off is acceptable and reduces magnetic property uniformity.
The substrate may be oriented relative to the flux direction so that its surface normal is aligned with the line of sight between substrate and the deposition flux source region on the sputter target, which is typically the center of the target, and rotated about its centerline. Under these circumstances, the substrate is not shadowed by the feature on the inboard or radially-innermost side of the feature. In contrast, the substrate will always be shadowed by the feature on the outboard or radially-outermost side of the feature. The degree of shadowing on the outboard side increases with increasing radial separation between the feature and the substrate centerline and also with increasing divergence of the deposition flux. The resulting deposition profile is highly asymmetrical.
Tilting the surface normal with respect to the line of sight between the target and the substrate during deposition improves the symmetry of the deposition profile by reducing the substrate shadowing on the outboard side of features. However, the nature of the substrate shadowing on the outboard and inboard sides of the feature depends on the azimuthal position of the feature as the substrate is rotated, as described below.
FIGS. 2A and 2B illustrate the shadow cast on a substrate 21 by the inboard side and the outboard side of a feature 26 projecting from substrate 21 at a location between the substrate center and peripheral edge. FIG. 2A shows the feature 26 with the substrate 21 oriented at a first azimuthal angle and tilted relative to a target 28 of an IBD system. The outboard side of the feature 26 shadows the substrate 21 over a distance 24. The inboard side of the feature 26 does not shadow the substrate 21. FIG. 2B shows feature 26 with the substrate 21 oriented at a second azimuthal angle that locates feature 26 at an angular position diametrically opposite to the position at the first azimuthal angle. The inboard side of the feature 26 shadows the substrate 21 over a distance 22, which is a smaller distance than distance 24. The outboard side of the feature 26 does not shadow the substrate 21 at the second azimuthal angle.
Despite substrate tilting, the shadowing of the substrate 21 over distance 24 on the outboard side of the feature 26 differs from the shadowing of the substrate 21 over distance 22 by the inboard side. In particular, the profile of the deposited material will differ on the inboard and outboard sides of the feature 26 adjacent to the sidewalls of feature 26. Specifically, the longer shadow cast over distance 24 adjacent to the outboard side results in a relatively longer taper of the deposited material than adjacent to the inboard side.
The shadowed substrate region on the outboard side of the feature 26 also experiences a lower deposition rate because it is effectively further away from the target 28 when the substrate 21 is oriented at the first azimuthal angle. The inboard substrate region experiences a higher deposition rate because it is closer to the target 28 when the substrate 21 is oriented at the second azimuthal angle. Therefore, the deposited material is thinner on the outboard side of feature 26, due to the outboard region being further away from the target 28. The asymmetry and difference in deposition rate, which originate from the beam divergence of the target 28, increase with increasing radial distance from the center of substrate 21.
Feature 30, which is at the same radial distance from the substrate center as feature 26, experiences the same asymmetries and differences in deposition rate as feature 26. On the other hand, the deposited material is radially symmetrical about feature 32 at the substrate center because feature 32 symmetrically shadows the substrate 21 adjacent to its sidewalls. Other types of surface treatments, such as etching, will have similar asymmetrical treatment profiles about the features 26 and 30.
It would therefore be desirable to provide a deposition method capable of eliminating or, at the least, significantly reducing the inboard and outboard asymmetries of the deposited material adjacent to a feature projecting from the surface of a substrate.